There is an increasing demand for communication devices capable of operating across a variety of different frequency bands. In particular, there is an increasing demand for cellular or mobile telephones that can operate in multiple frequency bands. In such devices, separate transmit and receive filters are in general employed for each transmit and receive frequency band. In practice, bulk acoustic wave (BAW) filters are often employed.
The simplest implementation of a BAW resonator comprises a layer of piezoelectric material arranged between two metal electrodes. Common piezoelectric materials are, for example, aluminum nitride (AIN) or zinc oxide (ZnO).
FIG. 1 shows an exemplary BAW resonator 10 having a static capacitance C which comprises a layer of piezoelectric material which will be referred to as piezo layer 12 below and is located between a first electrode, or top electrode T, and a second electrode, or bottom electrode B. The designations top electrode and bottom electrode merely serve definition purposes and do not represent any limitation with regard to the spatial arrangement and positioning of the BAW resonator. Rather, the designations top electrode and bottom electrode serve to define the positions of these electrodes in relation to a polarization of the piezoelectric material, as will be explained below, so that the polarization of the respective BAW resonators can be derived from an equivalent circuit diagram designating the T and B electrodes.
If an electric field is applied between first electrode T and second electrode B of BAW resonator 10, the reciprocal or inverse piezoelectric effect will cause BAW resonator 10 to mechanically expand or contract, the case of expansion or of contraction depending on the polarization of the piezoelectric material, as has been mentioned above. This means that the opposite case applies if the electric field is inversely applied between the T and B electrodes. In the case of an alternating field, an acoustic wave is generated in piezo layer 12, and, depending on the implementation of the BAW resonator, this wave will propagate, for example, in parallel with the electric field, as a longitudinal wave, or, as a transversal wave, transverse to the electric field, and will be reflected, for example, at the interface of piezo layer 12. Whenever the thickness d of piezo layer 12 and of the top and bottom electrodes equals an integer multiple of half the wavelength λ of the acoustic waves, resonance states and/or acoustic resonance vibrations will occur. The fundamental resonance frequency, i.e. the lowest resonance frequency FRES, will then be inversely proportional to total thickness of the resonator. This means that the BAW resonator vibrates at the frequency specified externally.
The piezoelectric properties and, thus, also the resonance properties of a BAW resonator depend on various factors, e.g. on the piezoelectric material, the production method, the polarization impressed upon the BAW resonator during manufacturing, and the size of the crystals. As has been mentioned above, it is particularly the resonance frequency which depends on total thickness of the resonator.
As has been mentioned above, BAW resonators exhibit electric polarization. The direction of mechanical deformation, extraction or contraction, of the BAW resonator depends on the direction of the electric field applied to first electrode T and second electrode B and on the direction of polarization of BAW resonator 10. For example, if the polarization of the BAW resonator and the direction of the electric field are pointing in the same direction, BAW resonator 10 contracts, whereas BAW resonator 10 expands when the polarization of BAW resonator 10 and the direction of the electric field are pointing in the opposite direction.
BAW resonators can have a variety of configurations. Typically, one differentiates between so-called FBARs (thin film bulk acoustic resonators) and SMRs (solidly mounted resonators). In addition, a BAW resonator may have one piezo layer 12, or it may have several piezo layers 12.
BAW resonators are often employed in filters.
FIG. 2 shows a circuit diagram of a filter 20, comprising a first series BAW resonator 48, a second series BAW resonator 55, a third series BAW resonator 52, a fourth series BAW resonator 53, a first shunt BAW resonator 54, a second shunt BAW resonator 56, a third shunt BAW resonator 58 and a fourth shunt BAW resonator 65. The series BAW resonators 48, 55, 52, 53 are connected in series between input port 44 and output port 46. First shunt BAW resonator 54 is connected in parallel between input port 44 and electrical ground 42. Second shunt BAW resonator 56 is connected between a connection node between first series BAW resonator 48 and second series BAW resonator 55, and electrical ground 42. Third shunt BAW resonator 58 is connected between a connection node between second series BAW resonator 55 and third series BAW resonator 52, and electrical ground 42. Fourth shunt BAW resonator 65 is between a connection node between third series BAW resonator 52 fourth series BAW resonator 53, and electrical ground 42. Each of the series and shunt BAW resonators comprises a top electrode T and a bottom electrode B, which are indicated in the equivalent circuit diagram of FIG. 2 so as to indicate the polarization of each of the BAW resonators.
One problem with filter circuits employing one or more BAW resonators, such as filter 20, is non-linear behavior of one or more of the BAW resonators. This problem occurs particularly when a BAW resonator is driven with higher power levels, and can result in the undesirable generation of harmonic content in the output signal.
One approach to mitigating this issue is to “cascade” the affected BAW resonators. In the following, a “cascade” means a chain, or series connection of elements. That is, a BAW resonator exhibiting static capacitance C is replaced by a cascade of two BAW resonators, each exhibiting a static capacitance 2C, so that the total capacitance of the series combination is again C. In principle, such a cascaded pair of BAW resonators has the same impedance properties as a corresponding individual BAW resonator. A cascaded pair of BAW resonators exhibiting static capacitance 2C is larger, by a factor of 4, than a corresponding individual BAW resonator exhibiting static capacitance C. As a result of the above-mentioned cascading, the energy density is also smaller by a factor of 4, and, thus, non-linear effects are reduced by 6 dB with a cascaded BAW resonator.
However, the aforementioned cascading arrangement has some drawbacks. A major disadvantage of replacing a BAW resonator by an equivalent cascaded pair of BAW resonators is the increase in size by a factor of 4, as noted above. Accordingly, such cascading considerably increases the size of the filter if it is carried out for all BAW resonators of a filter. As a result, it is generally impractical to utilize cascading for all resonator branches of a filter.
What is needed, therefore, is a general matching network and method of matching an antenna or other device to a plurality of BAW, SAW, and/or FBAR filters than can alleviate one or more of these shortcomings.